A cellular network comprises a group of base stations that defines the radio coverage areas (or cells) of the network. Typically, a non-line-of-sight (NLOS) radio propagation path exists between a base station (BS) and a subscriber station (or mobile station, mobile terminal, etc.) due to natural and man-made objects situated between the base station and the subscriber station. As a result, the radio waves propagate via reflections, diffractions and scattering. The arriving waves at the subscriber station in the downlink (and at the base station in the uplink) experience constructive and destructive additions because of different phases of individual waves. This is due the fact that, at the high carrier frequencies typically used in cellular wireless networks, small changes in the differential propagation delays introduce large changes in the phases of the individual waves.
If the subscriber station (SS) is moving or there are changes in the scattering environment, then the spatial variations in the amplitude and phase of the composite received signal will manifest themselves as time variations known as Rayleigh fading or fast fading. The time-varying nature of the wireless channel requires very high signal-to-noise ratio (SNR) in order to provide the desired bit error or packet error reliability.
Conventional wireless networks use various diversity techniques to combat the effect of fast fading. Diversity techniques provide the receiver (e.g., subscriber station) with multiple faded replicas of the same information-bearing signal. Assuming independent fading on each of the antenna branches, the probability that the instantaneous signal-to-noise ratio (SNR) is below a certain threshold on each of the branches is approximately pL, where p is the probability that the instantaneous SNR value is below the same threshold on each antenna branch.
Conventional diversity techniques generally fall into categories of space, angle, polarization, field, frequency, time and multipath diversity space diversity uses multiple transmit or receive antennas, where the spatial separations between the multiple antennas are chosen so that the diversity branches experience fading with little or no correlation. Transmit diversity uses multiple transmit antennas to provide the receiver with multiple uncorrelated replicas of the same signal.
Conventional transmit diversity schemes may be further divided into open-loop or closed-loop transmit diversity schemes. In an open-loop transmit diversity scheme, no feedback is required from the receiver. In one conventional closed-loop transmit diversity scheme, the receiver computes the phase and amplitude adjustment(s) that should be applied at the transmitter to maximize the received signal power at the receiver. In another conventional closed-loop transmit diversity scheme, referred to as selection transmit diversity (STD), the receiver provides feedback to the transmitter on antenna(s) to be used for transmission.
One well-known example of transmit diversity is the Alamouti 2×1 space-time diversity scheme. In this approach, during any symbol period, two data symbols are transmitted simultaneously from two transmit antennas. During a first symbol interval, the symbols transmitted from a first antenna (ANT1) and a second antenna (ANT2) are denoted as s(1) and s(2), respectively. During the next symbol period, the symbols transmitted from antennas ANT1 and ANT2 are −s*(2) and s*(1), respectively, where −s*(2) is the negative of the complex conjugate of s(2) and s*(1) is the complex conjugate of s(1). Signal processing in the subscriber station (SS) recovers the original symbols, s(1) and s(2). It is noted that the instantaneous channel gain estimates, g1 (for ANT1) and g2 (for ANT2), are required for processing at the SS receiver. Thus, separate pilot symbols are required for antennas ANT1 and ANT2 for channel gain estimation.
Another convention diversity technique commonly available in OFDM systems is frequency diversify. In an OFDM system exploiting frequency diversity, the subcarriers allocated for transmitting to a particular subscriber station may be uniformly distributed over the whole spectrum. For example, if an OFDM network allocates 64 out of N=512 subcarriers to a first subscriber station, the network may allocate every eighth subcarrier (SC) to the first subscriber station starting at the first subcarrier (i.e., SC1, SCS, SC17, . . . , SC505). Frequency diversity techniques are generally used for high mobility users and/or for delay-sensitive services.
Another conventional form of diversity is provided by Hybrid Acknowledgement Request (ARQ). Hybrid ARQ is a retransmission scheme whereby the transmitter sends the redundant coded information in small increments. In Hybrid ARQ, the transmitter first performs channel coding on an information packet P and then breaks the resulting coded bit stream into smaller subpackets (i.e., SP1, SP2, SP3, . . . ). The transmitter then transmits the first subpacket SP1 to the receiver.
The receiver initially tries to decode the entire information packet P using the first subpacket SP1. In case of unsuccessful decoding, the receiver stores subpacket SP1 and sends a NACK signal to the transmitter. After receiving the NACK signal, the transmitter transmits subpacket SP2. After receiving subpacket SP2, the receiver combines subpacket SP2 with the previously stored subpacket SP1 and tries to jointly decode the original information packet P. If decoding still fails, the receiver sends a NACK signal and the transmitter transmits additional subpackets (i.e., SP3, SP4, . . . ). At any point, if information packet P is successfully decoded, as indicated by a successful cyclic redundancy check (CRC), for example, the receiver sends an ACK signal to the transmitter.
Conventional networks also use beamforming techniques to transmit to multiple subscriber stations. The receiver in the subscriber station estimates the complex gains, g0, g1, . . . , gP, to be used from each transmit antenna of the base station. The base station uses these weights for transmission to the subscriber stations. However, the feedback information containing the complex gains represents a significant overhead and degrades the overall system spectral efficiency of the network.
U.S. patent application Ser. No. 11/327,799, entitled “Method And System For Introducing Frequency Selectivity Into Transmissions In An Orthogonal Frequency Division Multiplexing Network” and filed on Jan. 6, 2006, discloses a method and apparatus for artificially providing diversity in an orthogonal frequency division multiplexing (OFDM) wireless communication system. U.S. patent application Ser. No. 11/327,799 was incorporate by reference above. In the method and apparatus disclosed in application Ser. No. 11/327,799, diversity is artificially provided by generating a plurality of delayed symbols from a first symbol and then transmitting each of the delayed symbols from a different antenna. Each of the delayed symbols may also be scaled by a different gain factor.
In an adaptive cyclic delay diversity scheme, the delay values can be different for different subscriber stations depending upon the subscriber station's channel profile, velocity, and other factors. For example, a large delay value may be chosen for a high-speed subscriber stations requiring frequency-diversity benefit while a small delay value may be adopted for a low-speed subscriber stations that may potentially benefit from frequency-selective multi-user scheduling. Moreover, if the channel is already sufficiently frequency selective, a small delay value may be sufficient even for frequency-diversity mode transmission for high Doppler subscriber stations.
In an exemplary base station that implements adaptive cyclic delay diversity (ACDD) transmitter using (P+1) transmit antennas, the cyclic delay values on antenna ANT1 through antenna ANTP for each subscriber station m may be designated Dm1, Dm2, and DmP, respectively. A non-delayed signal (Dm0=0) is transmitted from the first antenna, designated antenna ANT0. In a more general form, different complex gains, g0, g1, . . . , gP, may also be applied to signals transmitted from different transmit antennas. The transmission of the same OFDM symbol from different antennas artificially provides frequency-selective fading. The frequency-selectivity may then be exploited by either using frequency-selective multi-user scheduling for low-speed to medium-speed subscriber stations or frequency-diversity for high-speed subscriber stations.
By using adaptive cyclic delay diversity (ACDD), the reception in the subscriber station receiver resembles multipath transmission from a single transmit antenna. The composite channel response, Hmc(k), on subcarrier k can be written as:Hmc(k)=Hm0(k)+Hm1(k)·e−j2πkDm1/N+ . . . +Hmp(k)·e−j2πkDmp/N  [Eqn. 1]where Hmn(k) is the channel response for subscriber station m on antenna n, and k is the subcarrier index. In this case, it is assumed that the complex antenna gains, g0, g1, . . . gP, are all unity.
Alternatively, the adaptive cyclic delay diversity operation may be performed directly in the frequency domain. A weight ofe−j2πkDmp/N  [Eqn. 2]may be applied to subcarrier k transmitted from antenna p to subscriber station m, where DmP is the cyclic delay value on antennas p for subscriber station m.
In one example of resource portioning in an OFDM network, a total of 512 OFDM subcarriers may be divided into eight (8) groups (or subbands) of 64 subcarriers each. A given subscriber station may be allocated one or more of these subbands. In an exemplary embodiment of adaptive cyclic delay diversity, the 512 subcarriers of a first OFDM symbol may be transmitted from a first antenna with no phase shift (i.e., no delay), while the 512 subcarriers of the first OFDM symbol may be transmitted from a second antenna with a delay of one sample period. A one sample delay results in a weight ofe−j2πk/N  [Eqn. 3]applied to the kth subcarrier. A phase shift of 2π/N is applied to the first subcarrier and phase shift of 2π is applied to the last subcarrier, respectively, where N=512. Therefore, the phase shift applied to each subcarrier increases linearly with the subcarrier index (i.e., from subcarrier 1 to subcarrier 512).
It is noted that a complete cycle of phase shifts from 2π/N to 2π happens over the whole bandwidth. The phase shift increments by 2π/N from one subcarrier to the next. The phase shift applied to the subbands transmitted from the second antenna happens in increments of 2πM/N, where M is the number of subcarriers in a subband. In case of a cyclic delay of D samples, D cycles of phase shift from 2π/N to 2π happen over the whole bandwidth. The benefits provided by cyclic delay diversity may be achieved by applying a different random phase shift to different subcarriers. The receiver obtains the benefits of frequency-diversity because different subcarriers combine constructively and destructively, depending upon the random phase shift applied.
Cyclic delay diversity as well as other forms of transmit diversity schemes, such as space-time diversity (or STD) suffer from performance loss in the case of correlated antennas or correlated channels because there is little or no diversity present in the channel that can be exploited. Additionally, the frequency-selectivity introduced due to delayed transmissions from multiple antennas in case of adaptive cyclic delay diversity (ACDD) may result in loss in performance relative to transmission from a single antenna with no transmit diversity. It is noted that ACDD actually translates spatial or antenna diversity into frequency-diversity. When there is no spatial or antenna diversity present due to correlated antennas, ACDD cannot create any frequency-diversity. However, the delayed transmissions from multiple antennas create frequency-selectivity without frequency diversity, which results in performance loss. The antenna correlations may result from closely spaced antennas, lack of scattering, or both.
Therefore, there is a need for improved wireless networks that implement adaptive cyclic delay diversity. In particular, there is a need for wireless networks that implement adaptive cyclic delay diversity in correlated antenna and correlated channels conditions without performance loss.